1. Field of the Invention
The present invention relates to an optical unit and an endoscope including the optical unit.
2. Description of the Related Art
Endoscopes are used for observation of an inside of a subject that cannot be observed from an outside. In the case of electronic endoscopes, in a distal end portion of an insertion portion, which is to be inserted into a body cavity, a CCD (charge coupled device) is incorporated together with an optical unit (image-formation optical system). Because a CCD has a relatively high sensitivity to infrared light, an optical unit including an infrared light cut-off filter is used for proper color reproduction.
On the other hand, endoscopes are used not only for observation but also for therapy. For example, with an endoscope apparatus including a laser therapy device, e.g., incision of a diseased site is performed using a laser. When therapy using laser light with an infrared wavelength is performed, light reflected from a site subjected to the therapy is very intense and thus cannot sufficiently be cut off by an infrared light cut-off filter, and a resulting observation screen is extremely bright, which may make observation of an object image difficult. Therefore, in the optical unit, a laser light cut-off filter that cuts off (blocks) laser light for therapy is arranged in addition to the infrared light cut-off filter.
Here, Japanese Patent Application Laid-Open Publication Nos. 9-54255, 10-113329 and 11-76146 each disclose an optical unit for an endoscope using a multi-layer interference film (cut-off coat) using the effect of interference of light as a laser light cut-off filter.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2009-93198 discloses that in, e.g., a general video camera, an optical system (optical unit) having predetermined specifications is used in order to reduce periodic pattern ghost generation attributable to a plurality of light-receiving elements included in an image pickup device being arranged in a predetermined periodic pattern.